1. Field of the Invention
The present invention relates to a mask for exposure, and a method of manufacturing the mask.
2. Description of the Related Art
In recent years, semiconductor devices such as an LSI have increasingly become further microfabrication, and the minimum size of a device pattern has reached as narrow as 90 nm. To realize such microfabrication, it is necessary to improve the resolution of an exposure system used in photolithographic process, and research of super resolution technology using a phase shift mask has been actively done. In a pioneering example of the phase shift mask, a λ/2 plate for shifting a phase is provided on a light transmission portion between light-shielding patterns, as disclosed in Patent Document 1.
FIG. 1 is a sectional view of a digging Levenson mask that is a type of the phase shift mask.
In a Levenson mask 3 of a conventional example, first and second light-shielding patterns (2a, 2b) are formed on a transparent substrate 1 with a gap, and a concave portion 1a where a side surface recedes from the first light-shielding pattern 2a is formed on the transparent substrate 1 of shifter portion A. By making the side surface recede (go backward) in this manner, the width for exposure light made incident obliquely with respect to the transparent substrate 1 widens, and bouncing of exposure light by the side surface (shown in a dotted line) of the concave portion 1a. 
In the Levenson mask 3, the depth of the concave portion 1a is determined such that a phase difference between exposure light having passed through the concave portion 1a and exposure light having passed through the transparent substrate 1, which is non-shifter portion B with no concave portion 1a, shifts by 180 degrees even. Consequently, exposure lights having passed through the transparent substrate 1 interfere to offset with each other on a light path near the boundary of shifter portion A and non-shifter portion B, contrast in this area is improved, and exposure pattern of high resolution can be transferred onto a wafer.
However, to realize it, the phase difference between the above-described exposure lights must be confirmed to be actually 180 degrees when manufacturing the mask 3.
From this viewpoint, Patent Document 2 discloses that patterns for evaluating phase difference, by which the phase difference is measured, are provided in addition to patterns for device, by which patterns for device are projected, an optical phase difference measurement system confirms that the phase difference of the patterns for evaluating phase difference is 180 degrees, and the phase difference of the patterns for device is thus guaranteed.
[Patent Document 1] Japanese Patent Laid-open No. 62-50811 publication (Kokoku publication)
[Patent Document 2] Japanese Patent Laid-open No. 9-258427 publication
However, since the patterns for evaluating phase difference are formed in a design rule same as that of the patterns for device in the method of Patent Document 2, luminous flux generated by the phase difference measurement system is bounced by the patterns for evaluating phase difference when the design rule is fine, and accurate measurement of phase difference becomes difficult.